The invention relates to a circuit arrangement for an integrated output amplifier with two essentially identical constant current sources, an integrated voltage amplifier with two inputs and two outputs and essentially identical output transistors coupled with these, as well as two essentially identical negative feedback networks between the output transistors and the correspondingly assigned input transistors of the voltage amplifier.
Such highly integrated circuits are particularly suitable for output amplifiers with a relatively low power output, such as those which can be used in hearing aids.
As is generally known, the gain normally changes greatly with the output current at very low output impedances if the output-side load is a largely inductive load with a DC resistance of 50 Ohm and an AC resistance at 800 Hz of around 100 Ohm.
Such a circuit is known, for example, from U.S. Pat. No. 4,085,382. Here, a directly coupled integrated class B amplifier with a low power rating is described, whereby this consists of a three-stage two-channel pre-amplifier and a pair of output transistors, one for each channel. A negative feedback loop is provided in each channel which connects the collector of the transistor in the last stage of the preamplifier with the base electrode in the first stage of the pre-amplifier and thus controls the direct current level.
A negative, resistive AC voltage feedback loop connects the collector of one output transistor in each case with the collector of the corresponding first pre-amplifier transistor in order to reduce the gain dependence of the respective channel on the current flowing through the output transistor. This is intended to reduce the closed circuit currents in the output transistors.
However, these amplifiers have a relatively high gain of more than 40 dB.
However, this circuit is not without problems. Firstly, it requires too many and too large external components, e.g. two capacitors in the microfarad range and one capacitor in the nanofarad range for stabilization.
In addition, the power requirement is too high and the output power obtained from this is too low. Although it is always attempted to keep the closed-circuit current of the output transistors as low as possible, high distortions are obtained for a load with low impedance if this current is reduced too much. The better the output transistors are driven into the saturation region, the lower the amount of power tapped from the voltage source which is lost in the amplifier.
In the known circuit, the output transistors are controlled passively, i.e. by a resistor, and the level is kept low in order to limit the closed-circuit current. However, the result of this is that the saturation of the output transistors is insufficient. In addition, although the input-side noise is low with the known amplifier, the overall gain factor is high (greater than 40 dB), and this means in turn that the signal-to-noise ratio is not very good at a low gain setting.